1. Field of the Invention
The present invention relates to a vehicle equipped with a pair of transmissions, e.g., a pair of hydrostatic transmissions (HSTs), a pair of belt type continuous variable transmissions (CVTs), or a pair of electric motors, for driving respective left and right drive wheels, wherein the pair of transmissions, which surely drive the vehicle straight when they are set so, are operated for left and right turning of the vehicle so that one of the left and right drive wheels is accelerated and the other is decelerated, and wherein each of the left and right turning directions of the vehicle always coincides with the manipulated leftward or rightward direction of a steering operation device whether the vehicle travels forward or rearward.
2. Related Art
There is a well-known vehicle having twin parallel transmissions, such as HSTs, for driving respective left and right drive wheels. For left and right turning of the vehicle, one of the transmissions accelerates the corresponding drive wheel (on an outside of turning), and the other decelerates the other drive wheel (on an inside of turning). For example, if a steering wheel serving as a steering operation device of the vehicle is turned left, one transmission decelerates the left drive wheel, and the other accelerates the right drive wheel, whereby the vehicle turns left.
Incidentally, if the deceleration force applied on the inside drive wheel becomes relatively larger than the traveling force thereof, the rotational direction of the inside drive wheel is reversed. Then, the deceleration force becomes an acceleration force for the rotationally reversed inside drive wheel.
Also, unless the steering wheel is operated for turning, the traveling speed and direction of the vehicle can be changed by operation of a speed change operation device such as a pedal or a lever so that both the transmissions drive both the drive wheels at equal rotary speed. If the steering wheel is turned for turning, the rotary speed of one drive wheel becomes higher than the equal rotary speed, and that of the other drive wheel becomes lower than the equal rotary speed.
However, the vehicle having the twin transmissions still has such a problem as follows: When it is assumed that the left drive wheel is decelerated and the right wheel accelerated by turning the steering wheel leftward during the forward traveling of the vehicle, the steering wheel must be turned rightward for left turning during the rearward traveling of the vehicle because the acceleration force for a forwardly rotating wheel serves as a deceleration force for a rearwardly rotating wheel. Such an operation of the steering wheel is so illusional as to cause the vehicle to turn to a wrong side.
As means for coincidence of turning side of a vehicle with a steering direction whether the vehicle travels forward or rearward, Japanese Laid Open Gazette No. 2000-25,637 discloses mechanical means interposed among a transmission, a steering operation device and a speed changing operation device, and Japanese Patent No. 3,176,237 discloses a hydraulic switching valve provided in an HST for steering. However, while each of the two Japanese documents discloses a pair of HSTs for driving two rotors, one of the HSTs is provided for traveling so as to rotate both the rotors at various equal speeds, and the other HST is provided for steering so as to accelerate one rotor and decelerate the other rotor. Therefore, each of the above-mentioned disclosed means does not correspond to the vehicle driving system comprising a pair of transmissions for driving respective right and left drive wheels. Furthermore, while the rotors disclosed in Japanese Laid Open Gazette No. 2000-25,637 are drive wheels, those disclosed in Japanese Patent No. 3,176,237 are sprockets.
For another problem of the vehicle comprising the twin transmissions for driving respective drive wheels, even if the vehicle is provided with a driving direction changing system which ensures that one drive wheel on inside of the turning vehicle is decelerated and the other on outside thereof is accelerated in correspondence to the turning side of the steering operation device such as a steering wheel whether the vehicle travels forward and rearward, the performances of the parallel transmissions must be strictly equal to each other so as to ensure a straight traveling when the steering wheel is set in the straight traveling position. If there is a slight performance difference between the transmissions, the drive wheels rotate at different speeds so as to turn the vehicle while the steering wheel is in the straight traveling position, or a complicated and troublesome adjustment of a linkage from the steering wheel to the transmissions must be performed. Moreover, the differential driving of left and right drive wheels is not requested in such a case where the vehicle travels on a rough road or one of the drive wheels slumps into mud.
For solving this kind of problem, U.S. Pat. No. 5,590,737 discloses a crawler vehicle equipped with a driving system comprising a pair of HSTs for driving respective right and left sprockets, wherein a differential-locking clutch is provided so as to lock the sprockets with each other at need. However, this U.S. patent does not disclose a pair of transmissions for driving respective right and left drive wheels.